The pulp and paper industry is a highly water-intensive industry consuming enormous amount of water per ton of paper produced. The worrying factor is that it discharges approximately 70% of the water consumed during processing. However, this water discharge is no longer the clean water that was used for processing. Instead, it contains high amounts of organic as well as inorganic compounds, which emanate from the raw materials as well as the chemicals used during processing and production of paper. Due to the above, there is a sharp increase in the overall pollutional load of the wastewaters and subsequently, the receiving waters. It is for these reasons, the pulp mill industrial wastewaters have attracted the attention of environmentalist and people, globally.
Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are indices of the biologically degradable and chemically oxidizable fractions of the wastewater, respectively. These are monitored regularly to give clearance to the discharge of the above wastewaters. Total Dissolved Solids (TDS) is a parameter, which gives us the index of dissolved compounds, both organic as well as inorganic, present in the said wastewaters (APHA, 20th ed.). It is this dissolved strength of chemicals, which imparts a toxic load to the effluents and hence, the overall pollutional load. However, this parameter has long been ignored because of lack of suitable technologies available for the same.
Total Dissolved Solids need to be monitored regularly in wastewaters since such load changes the quality and composition of the receiving waters as well, which could be deleterious in many ways, besides creating aesthetic problems. Dissolved solids have a direct impact on various parameters like hardness, heavy metal content and carcinogenicity of the receiving waters leading to imbalances in aquatic biota and reduced water quality for domestic use.
A recently conducted survey on the Nam Phong river in Thailand indicated that it was much affected by the contaminated waters which flowed from the nearby Phoenix Pulp and Paper company and reportedly killed many fish. Evaluation of these waters revealed that besides the BOD and COD load, the TDS content was very high (Immuong, 1998). Hence, even though this parameter may not seem important as far as the implementing agencies are concerned, yet the implications of its high loading are manifold.
Pulp and paper mills employ various raw materials and chemicals for processing and production. As a result, the above chemicals, in their varied forms, are present in the discharges emanated from these mills and contribute to the dissolved solids content. The organic dissolved compounds consist mainly of chlorinated compounds, which arise in the form of chlorinated lignin derivatives, chlorinated phenolics and chlorinated resin acids like abietic and pimaric acids, etc. Organically bound halogens (or halide-OX) where the halogen is usually chlorine, imparts, what is called AOX (Adsorbable Organic halide) in the effluent (berry, 1992). Besides the above chlorinated compounds, unsaturated fatty acids and degraded oxidation products of celluloses and carbohydrates are also responsible for the increase in the TDS load.
Among the inorganic dissolved components, free chlorine, sulphates, sulphides, carbonates, bicarbonates, predominate as the major anions; and calcium, magnesium, aluminium, iron and other heavy metal ions as the prevalent cations. Both inorganic and organic dissolved solids raise the TDS (mg/l) to a very high level. In India, the Bureau of Indian standards (BIS) have set up an upper limit of 2100 mg/l TDS for discharges into rivers and streams. However, if evaluated properly, the pulp and paper effluents do not conform to the above standards, not withstanding the fact that a TDS level above 1200 mg/l is considered to be toxic to the aquatic system (USEPA). However, since the available and currently used TDS reducing technologies are not able to practically reduce the TDS to a great extent, the governmental agencies have not lowered the upper limit.
Currently available TDS reducing technologies are strictly physico-chemical in nature. The major technologies being Reverse Osmosis (RO), Electrodialysis Reversal (EDR) and Ion Exchange.
RO
It is a physical process in which contaminants are removed by applying pressure on the feed water to direct it through a semi-permeable membrane. This process requires a careful review of raw water characteristics and pretreatment to prevent membranes from fouling, scaling or other membrane degradation. This technology is relatively expensive to install and operate, requires frequent membrane monitoring and maintenance as also stringent pressure, temperature and pH requirements to meet membrane tolerance
EDR
It is an electrochemical process in which ions migrate through ion-selective semi-permeable membranes as a result of their attraction to two electrically charged membrane surface. EDR uses the technique of regularly reversing the polarity of the electrodes, thereby freeing accumulated ions on the membrane surface. However EDR is not suitable for high levels of Fe and Mn, H2S, chlorine or hardness. Also, the process is limited to water with 3000 mg/l TDS or less.
Ion Exchange
Based on the ionic charges of the components, ion-exchange resins trap the dissolved constituents and reduce the TDS. However, the problems of reusability of these ion-exchange columns increase the economic burden.
All the above physico-chemical techniques have the major disadvantages of economics, reusability and treatability range. Disposal problems are also there since these methods do not eliminate TDS completely and accumulate such solids elsewhere, in some other form.
Pulp mill effluents are markedly different in their nature and composition and hence the dissolved solids present therein are also different from those present in other effluents. Though biological treatment methods are always advantageous over the physico-chemical ones, TDS in pulp mill effluents is not effectively reduced by the conventional biological treatment. The overall pollution load, too, cannot be taken care of, by the above treatment, since, these effluents are typically deficient in nitrogen and phosphorus. Therefore, it becomes imperative for ETP operators to add supplementary nutrients, such as urea and phosphoric acid during treatment of such effluents. An overdosing of such nutrients is always done to some degree to ensure sufficient nitrogen demand under all conditions. As a result, treated wastewater, usually contain excess amounts of both nutrients, contributing to the overall dissolved solids contents and potential impacts on the receiving waters such as eutrophication (DMEHIA, 1999).
It is because of all the above reasons, the inventors felt the need for developing a biological treatment technique which would reduce the TDS levels in an inexpensive and environment-friendly manner. It was felt that bacteria isolated from natural environment, would be capable of reducing the level of TDS in wastewaters. Initially, consortia of bacteria were studied but later it was observed that a single bacterium is also equally capable of the same.